This invention pertains to a device and method for determining the revolutions of a spin stabilized projectile and subsequently arming a safety and arming device a fixed number of turns after firing.
In spin stabilized projectiles such as artillery shells, it is desirable that arming of the projectile takes place at a specific distance from the gun barrel. One method used in the prior art is a fixed time delay. After a predetermined fixed time, the projectile is armed. Because artillery shells from different sized guns, as well as the same gun can have widely differing velocities, the arming distance of the shells will undesirably vary with the velocity thereby creating a potentially dangerous situation.
To overcome this problem, devices have been developed which determine the distance the projectile has traveled by detecting the number of turns made by the projectile. Since the distance and rotation of the projectile are related linearly, this provides a distance-sensing function indirectly. Many of the devices presently in use employ mechanical means to determine the distance the projectile has traveled. U.S. Pat. No. 3,853,062 entitled "Device for Measuring Distance of Travel by a Projectile" issued Dec. 10, 1974, uses a pendulum inside the fuze of a spin stabilized projectile. One complete swing of the pendulum is equal to or proportional to one complete revolution of the spinning shell. The distance traversed by the projectile is determined from reading the number of complete cycles made by the pendulum which, in turn, is equal to the number of revolutions of the spinning shell. While this device and other mechanical devices of like nature produce the desired results, they are mechanically complex and have moving parts which increase the chance of failure and increase the cost of the device.
Electromechanical devices have also been developed which determine the distance the projectile has traveled and set a time delay. Some of these devices employ digital threshold acceleration switches in conjunction with a microprocessor. While these devices can determine the distance traveled by the projectile with a varying degree of accuracy, the device has moving parts and is complex causing increased cost and reliability problems.